Sludge from municipal waste water treatment plants or paper mills is typically passed through a mechanical dewatering operation to remove a portion of the water. The mechanically dewatered sludge is then fed to a sludge drying plant which utilizes heated air in a suitable drying drum. The dried sludge is then pelletized and disposed of as permitted by the local regulations.
Municipal waste water sludges generally contain a high portion of organic components which must be decomposed before disposal. Typically, the sludge is passed through a heated drier to inactivate or decompose the organic components. Several countries have enacted regulations which are expected to be effective in 2005 to restrict the organic content of the sludge before it can be transferred to a landfill. These regulations generally require an organic content of less than 5% before disposal.
At this time, waste water sludges, and particularly sewage sludges, are passed through a series of heated drying stages after the mechanical dewatering step to ensure decomposition of the organic materials and complete drying of the sludge. Sewage sludges can be particularly difficult to handle since the sludge goes through a sticky phase during drying. Typically, the drying process for sewage sludge recycles a portion of previously dried material and mixes the dried material with fresh wet sludge to increase the solid content of the material fed to the heated drier. Examples of this type of process are disclosed in U.S. Pat. No. 5,318,184 to Krebs, U.S. Pat. No. 5,309,849 to Krebs, U.S. Pat. No. 5,611,935 to Theimann et al. and U.S. Pat. No. 5,337,496 to Glorioso, which are hereby incorporated by reference in their entirety.
Backfeeding of recycled dried material is costly and reduces the output of the drying apparatus. Thus, efforts have been made to reduce the amount of backfeed material. For example, German Patent DE 4332799 discloses a drying chamber surrounding a centrifuge. This design however is difficult to maintain and very expensive to manufacture. Furthermore, this design of the centrifuge is difficult to operate in a manner to produce a uniform output of dried material and a uniform granular size. As a result, subsequent drying steps produce particles of uneven moisture content and non-uniform particle size.
Accordingly, a continuing need exists in the industry for a drying process and apparatus for drying municipal waste water sludges.